How to Make Humidity Disks for Herb Jars

Simple homemade clay humidity disks (a.k.a. “Grubbycoins”) can be used to extend and improve the life of dried herbs and spices by helping regulate humidity.

Clay particles when bisque fired will fuse together enough that they will no longer separate in water, but the overall structure remains porous enough to absorb or release moisture. Bisque clay, when dipped in water, will soak up moisture like a rigid sponge. Then it can be put into a sealed container where it will slowly release moisture and raise the humidity of the interior. This can help slowly rehydrate overly dried material. This is similar to practices where a slice of bread or similar is used but without the disadvantage of adding a fungal food source.

Although to be fair, a piece of broken terracotta flowerpot works just as well, it isn’t as much fun as making your own:

Making Humidity Disks

1) Obtain some natural clay.

Do not use a plastic or oil clay, it has to be the type of clay intended to be fired in a kiln. Since a glaze won’t be used (that would clog up pores) to add decoration I used two different colors of cone 5 clay in this example. Previously I’ve also used unmarked natural clay and prepared clay from the riverbank with success, so there is a fair amount of flexibility in the exact choice of clay.

2) Form the humidity disks.

A rolling pin and cookie cutter can speed the process.

There are a lot of different ways to go about this part. As long as the end result is a disk or other simple shape an inch or two (24-50mm) in diameter and a quarter-inch (5 mm) or so thick it should work fine.

Here are a few ideas:
They can be formed entirely by hand.
Or rolled out and cut out like cookies.
Use layers of different colored clay to form a log, and slice with a wire cheese cutter or thin string Nerikomi style.

3) Leave the disks out for a day.

Or so until they have completely dried. They are at their most fragile at this point so use care to reduce breakage.

4) Place on a cookie sheet and put into a cold oven.

Warm in the oven for 1-2 hours at 150-180 F (65-82 C). Do not let temperatures reach the boiling point of water. The intent is to make absolutely sure all the free water is evaporated out of the disks. If there is any free water inside the clay and it is allowed to boil the steam pressure can break or rupture the clay disk.

5) Turn the heat up.

The next step in heating will drive off the remaining water locked in the clay. Turn up the heat to a little over the boiling point of water, and then slowly increase the temperature to 350 F (176 C) over the next couple of hours.

6) Turn the heat down.

Turn the oven down, and allow the humidity disks to cool.

7) The disks can then be either taken to a kiln for firing or fired in a charcoal BBQ.

To fire in a BBQ. Stack the coins in the BBQ between layers of a pile of charcoal. Leaves and salt can be added Raku style to add effects, but be careful not to use any toxic colorants. Light the briquettes and allow them to burn until they burn out and cool.

9) The next day, retrieve the humidity disks from the ashes, rinse with clear water and use or give as gifts.

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Grubbycup was raised on a family-operated organic dairy farm in central California.